The present invention relates to an improved process for the manufacture of aromatic amines by catalytic hydrogenation of nitroaromatics in the gas phase and to a new catalyst useful in this process.
Anilines are important intermediate products for the manufacture of dyestuffs, polyurethanes and plant-protection products.
Various methods for hydrogenation of nitrobenzene and other nitroaromatics are known. Due to the large enthalpy of reaction released during these known processes, all are carried out in reactors having integrated heat-carrier systems. For example, hydrogenation in liquid phase on a suspended catalyst (e.g., Pd catalyst) is described in EP 476,404. Hydrogenation in the gas phase on a fluidized solid catalyst is disclosed, for example, in U.S. Pat. No. 3,136,818. Hydrogenation in the gas phase on a stationary catalyst (e.g., a supported Pd catalyst) is described in DE-A 2,244,401; 2,849,002; and 4,039,026.
In DE-A 2,244,401 and 2,849,002, Pd catalysts on aluminum oxide supports that can be operated as stationary catalyst beds in heat-exchanger tubes under normal pressure at loadings of less than 1 g nitrobenzene (Nbz)/ml catalyst h with low hydrogen/nitrobenzene ratios are described. Between 6 and 11 moles of hydrogen per mole of nitrobenzene are fed into the reactor. These catalysts must be regenerated approximately every 1000 hours. Each of the described processes is operated in a Gas Hourly Space Velocity (GHSV) range between 600 and 900 h.sup.-1.
In DE-A 4,039,026, Pd catalysts on graphitic supports are described. The process described in this disclosure is carried out under conditions similar to those used for processes in which Pd catalysts on aluminum oxide are used. At loadings clearly below 1 g (Nbz)/ml catalyst h and a hydrogen/nitrobenzene ratio of 14-26 moles to 1 mole, the catalysts show incomplete conversion. Between 1000 and 4000 ppm nitrobenzene, relative to aniline formed, are found in the condensate. The selectivities relative to aniline vary between 99.1 and 99.6%. The process is described for a GHSV range between about 2000 and 3150 h.sup.-1.
Both an increase of the nitroaromatic loading and a raising of the hydrogen/nitroaromatic ratio raise the volumetric flow rate through the catalyst bed thus reducing the residence time on the catalyst. Both measures should therefore lead to an increase of the nitroaromatic breakthrough (i.e., incomplete conversion).
A general measure of this gas flow through the catalyst bed is the Gas Hourly Space Velocity (GHSV), quoted in the unit h.sup.-1.
However, even small amounts of nitroaromatics in aromatic amines lead to distinct discoloration of the otherwise colorless aromatic amine and are therefore undesirable. Removal of the nitroaromatics by distillation is demanding, both with respect to the apparatus and to the amount of energy consumed.
In each of these process variants, the large heat of reaction generated must be withdrawn from an industrial reactor via a complicated heat carrier system.
Hydrogenation processes in the gas phase with simple adiabatic catalyst beds are particularly economical because simple apparatus without an integrated heat exchanger system may be used. However, the large exothermicity of nitro group hydrogenation means that special requirements for the catalyst must be met. The catalyst must hydrogenate selectively over a wide temperature range. In the adiabatic process, a heat carrier (in hydrogenation processes this carrier is usually hydrogen) is also admixed with the educt mixture which leads to very short residence times (i.e., large GHSV's). The catalyst must therefore, in addition to its selectivity over a wide temperature range, be very active in order to obtain a complete nitrobenzene conversion, even at low nitrobenzene loadings.